A foreign company which manufactures electric bulbs has assured its customers that the lifespan of the bulbs is 28 month. Recently the company embarked on a quality improvement research for their product. After the research using new technology, a sample of 70 bulbs was tested and they gave a mean lifespan of 30.2 months with a standard deviation of 4months. Does this justify the research undertaken? the critical value of the one tailed test at 1% level of significance to conduct a statistical test in order to establish the truth about the above question + 2.33